Possibility to add multiple form keys

phpBB uses the add_form_key function to secure form submissions. The function assigns the template variable S_FORM_TOKEN (here) which the template then displays. However, when extensions want to secure their form submissions on pages where phpBB already called that function, it won't work because it overwrites the already assigned template variable.

For this case I'd like to have a way to add a template variable suffix:

'S_FORM_TOKEN' . $template_variable_suffix => $s_fields,

It could be passed via an optional function argument $template_variable_suffix = '' or it could be set in the core.add_form_key event.

Did I miss anything or shall I prepare a PR for this?